1. Field of Invention
The present invention pertains to the field of sensors providing information to automatic transmission controllers for motor vehicles and, more particularly, to a method and system for detecting whether a door is open or closed and if a door sensor fails.
2. Background of the Invention
A traditional automatic transmission includes a control device employed to control the transmission of a motor vehicle. In particular, the transmission control device is used to select several ranges, such as Park wherein the transmission is locked to prevent the vehicle from moving, Neutral wherein the transmission allows the vehicle to be moved freely, such as when being towed, Reverse wherein the transmission allows the vehicle to move backwards, and one or more Drive ranges that enable forward motion of the vehicle. Usually, the transmission control device is in the form of a lever connected with a mechanical connection, such as a cable or a hydraulic line, to the transmission. Typically, the lever is also connected to an indicator. As the transmission control mechanism is moved from one range to another, the mechanical connection physically shifts the transmission to the selected setting and the indicator moves to show the driver which range has been selected. Even if the vehicle is turned off, the driver is able to determine the current transmission range from the indicator and, in some cases, to move the transmission control mechanism to Neutral if for example, the vehicle is to be towed.
The traditional automatic transmission utilizes multiple friction elements for automatic gear ratio shifting. Broadly speaking, these friction elements may be described as torque establishing elements, more commonly referred to as clutches or brakes. The friction elements function to establish power flow paths from an internal combustion engine to a set of vehicle traction wheels. During acceleration of the vehicle, the overall speed ratio, which is the ratio of a transmission input shaft speed to a transmission output shaft speed, is reduced during a ratio upshift as vehicle speed increases for a given engine throttle setting. A downshift to achieve a higher speed ratio occurs as an engine throttle setting increases for any given vehicle speed, or when the vehicle speed decreases as the engine throttle setting is decreased. Various planetary gear configurations are found in modern automatic transmissions. However, the basic principle of shift kinematics remains similar. Shifting an automatic transmission having multiple planetary gearsets is accompanied by applying and/or releasing friction elements to change speed and torque relationships by altering the torque path through the planetary gearsets. Friction elements are usually actuated either hydraulically or mechanically based on the position of the transmission control device.
In a shift-by-wire transmission arrangement, the mechanical connection between the transmission control device and the transmission is eliminated. Instead, the transmission control device transmits an electrical signal along a wire to an electronic controller, which directs separate actuators to apply or release the various friction elements to obtain a desired gear ratio. The control device is no longer necessarily in the form of a lever because the control device is no longer moving a mechanical connection for controlling the transmission. Instead, the control device is typically an electro-mechanical interface (e.g., a series of buttons, lever or knob) that is used to instruct the transmission to switch between the transmission ranges. An electronic display, powered by a battery on the vehicle, is typically employed to indicate the current range for the transmission and must be on, and thus drawing power, in order for the driver to know which range has been selected.
Many vehicles with a shift-by-wire transmission incorporate a “Return to Park” feature to automatically shift the transmission into Park when the driver exits the vehicle or the battery supplies a voltage below a certain threshold level. Automatically shifting the transmission into Park prevents unwanted motion of the vehicle. See, for example, U.S. Pat. Nos. 3,937,105, 4,892,014 and 7,156,218. Such a feature is activated when certain triggering events occur, for example, when the system detects a seat belt being unbuckled while a driver door is opened and the vehicle is essentially stationary, or when the ignition is turned off. Sensors or switches are typically used to detect the triggering events. When these sensors, which are preferably in the form of switches, fail, the “Return to Park” functions do not operate properly and in some cases the “Return to Park” functions are disabled. Some controllers that normally use sensor inputs to control a transmission will ignore a sensor that is known to be faulty and control the transmission based on the remaining sensors. For example, when the controller detects a faulty door sensor, the controller will ignore the door sensor and control the transmission based on a seatbelt sensor. The seatbelt sensor may be a hall sensor connected to a control system incorporating diagnostic routines that determine if the sensor is sending a faulty signal. However, in the case of a driver's door latch sensor used to determine if the driver's door is open, the sensor may have two valid states, i.e., door open or door closed. Since both states are valid, determining if the sensor has failed simply by looking at its state is not possible. If the switch fails and always sends a signal indicating that the door is closed, even when it is not, then the “Return to Park” feature becomes disabled.
Solving this problem is surprisingly difficult. Proposed solutions have proved to be ineffective or costly. For example, setting a door state within the controller to “indeterminate” until a signal is received from the door latch sensor indicating a transition from open to closed or vice versa may seem promising but causes unexpected problems. With such a system, a driver might be at car wash and turn off the engine to conserve fuel, which might also power off the controller, thus setting the door state to indeterminate and causing the controller to use only the seatbelt sensor. Then later, as the driver restarts the car and drives into the carwash and selects neutral, any unbuckling of the seatbelt will cause the “Return to Park” function to operate unexpectedly. Another proposed solution replaces the simple switch to one with inherent diagnostic capabilities. However, this solution uses an expensive switch and is thus undesirable.
As can be seen by the above discussion, there exists a need for a method and system for detecting a door state and door sensor failures that is cost effective and does not result in the transmission control system behaving in unexpected ways.